1. Field of the Invention
The present invention relates to a frequency synthesizer for generating signals having a plurality of desired frequencies, and also relates to a multi-band radio apparatus using the same.
2. Description of the Related Art
In general, mobile communication terminals are designed for the purpose of using in one communication system. Such a communication system may be a PDC (Personal Digital Cellular) mobile phone system, a mobile phone system conforming to IS-95, or PHS (Personal Handy-phone System). It is quite usual that one mobile communication terminal complies with only-one standard among others of various communication systems existing in the world.
Recently, demand is raised to provide another mobile communication terminal to cope with rapid diversifications of the mobile communication systems. Such a terminal can solely control transmission/reception in response to multiple, different communication systems. For example, so-called “multi-mode terminal” used for both the PDC mobile phone system and the PHS has already been proposed.
In most cases, different mobile communication systems use different frequency bands; therefore, a multi-mode terminal to deal with them should be provided with a “multi-band radio function”, i.e., a function of transmitting/receiving data within each of multiple frequency bands.
A direct conversion mode is known as an architecture suitable for realizing such a multi-band radio apparatus. In the apparatus using the direct conversion mode, received signals from an antenna are inputted to one of quadrature demodulators.
To the quadrature demodulator, a pair of local signals for receiver having phases different from each other by 90° are also inputted. They are generated by subjecting local signals output from a frequency synthesizer, to the phase shift by a π/2 phase shifter. Note that frequencies of the local signals are set with regard to frequencies of desired signals in the received signals.
Because the quadrature demodulator multiplies the received signals by the local signals, the desired signals are converted into baseband signals for an I (Inphase) channel and a Q (Quadrature phase) channel with a center frequency of 0 Hz, which are inputted to a baseband reception section for subsequent signal reproduction processing.
On the other hand, signals to be transmitted for the I channel and the Q channel generated by a baseband transmission section are inputted to the another quadrature modulator.
To a local input port of the quadrature modulator, local signals for transmitter having phases different from each other by 90°, which are generated by subjecting local signals output from the frequency synthesizer to the phase shift by the π/2 phase shifter are inputted.
Frequencies of the local signals are set to be equal to a transmission frequency. As this quadrature demodulator multiplies the transmission signals by the local signals, the frequencies of the transmission signals are converted into a predetermined transmission frequency.
The frequency synthesizer used in the multi-band radio apparatus must generate local signals in various frequency bands according to realization of the multi-band. Note that this requirement is not limited to the direct conversion mode.
Various modes such as GSM (global system mobile communication) using the 900 MHz band, DCS (digital cellular system) using the 1800 MHz band, PCS (personal communication services) using the 1900 MHz band, UMTS (universal mobile telecommunication system) using the 2 GHz band are extensively utilized in the world. Development of a four-band radio apparatus supposed to be used in all of these frequency bands is desired.
When the frequency synthesizer to cope with such a four-band radio apparatus is realized in compliance with, for instance, the direct conversion mode, there can be considered a method for preparing respective unit synthesizers for: GSM transmission, GSM reception, DCS transmission, DCS reception, PCS transmission, PCS reception, UMTS transmission and UMTS reception by analogy with the method for constituting the frequency synthesizer in the two-band radio apparatus which can cope with both PDS and PHS.
Since the reception frequency band of PCS and the transmission frequency band of UMTS are nearly equal to each other, one synthesizer can function for the both modes. That is, except special cases, unit synthesizers whose number corresponds to a plurality of necessary frequency bands are basically prepared. Therefore, when a number of bands is increased, a number of the unit synthesizers is also proportionately increased, which results in vast hardware.
In preparing the unit synthesizers according to the respective frequency bands in order to realize the multi-band radio apparatus, multiple unit synthesizers are required when a number of bands is increased. Therefore, the scale of hardware become larger, which leads to increase in size of the multi-mode terminal and the price and the power consumption.